TiO2 has been the dominant semiconductor photocatalyst, although there are many other types of semiconductor photocatalysts. The domination of TiO2 in the field can be attributed to its superior photocatalytic oxidation ability, as well as its non-photocorrosive, non-toxic and inexpensive characteristics and can be readily synthesized in its highly photoactive nanoparticle forms. In practice, different applications may well require photocatalysts with different photocatalytic characteristics. These characteristics are known to be determined by the structural, compositional and morphological parameters of the material, which can be manipulated via different synthesis methods under different conditions.
Over the past 20 years, many synthesis methods have been developed for fabrication of different forms of TiO2 photocatalysts. The Sol-gel method, electrochemical anodization method, liquid template method and various hydrothermal methods are the most widely used synthesis methods. Among these methods, sol-gel method is the earliest and most well-studied method for synthesis nanoparticulate TiO2 photocatalyst. It has been used almost exclusively to obtain the nanoparticulate form of TiO2.
The electrochemical anodization method was first reported in 2001. The method is capable of achieving large scale highly ordered and vertically aligned TiO2 nanotubes via a simple one step electrochemical process. The subsequent thermal treatment results in highly photocatalytic active forms of TiO2 nanotubes suitable for a range of applications. The attraction of such a form of TiO2 photocatalysts lies in their unique dimensional structure, rich source of new physicochemical properties, and their enormous application potential to various fields. It has been widely reported that utilising a vertically aligned nanotubular TiO2 photoanode can increase the photocatalytic efficiency of water cleavage and dye-sensitized solar cells. The mechanistic basis of photocatalytic efficiency enhancement has been attributed to the effective electron percolation pathway provided by the highly ordered perpendicularly aligned nanotubular architecture. For a nanoparticulate system, the structure disorder at the contact between nanoparticles increases the scattering of free electrons and therefore reduces electron mobility. Consequently, the electron transport is often the limiting factor of the overall photocatalytic process.
The liquid template method is a diversified method that covers a very broad range of different templates and based on very different mechanisms. Different forms of TiO2 nanostructures (e.g. nano-planar, nanotubular, mesoporous, highly ordered and patented arrays) can be obtained by this method. Hydrothermal methods have been around for many years but only recently being employed for synthesis of nanostructured TiO2. The method can be used to synthesise various forms of TiO2 including nano-planar, nanotubular, nano-fibre and mesoporous forms.
U.S. Pat. No. 5,525,440 discloses forming a photo-electrochemical cell in which an initial layer of titanium oxide is formed and annealed on a conductive glass as a porous layer and then a non porous titanium oxide layer is applied and then finally a further porous titanium oxide layer is applied and the whole electrode is then annealed at 500 C. The electrode is then subjected to a further titanium oxide electrochemical deposition.
U.S. Pat. No. 6,281,429 discloses a transparent electrode of titanium dioxide on ITO glass and is formed at a thickness determined by a particular formula.
Japanese abstract 2004196644 discloses a forming a titanium dioxide film from a sol and then sintering it.
Japanese abstract 59121120 discloses a reduction in vacuum treatment for titanium dioxide to improve its efficiency.
U.S. Pat. No. 629,970 discloses a method of forming a semiconductor oxide in which the nano particles are first formed by precipitation, heated in the range of 250 C to 600 C, then dispersed and coated on a surface and then treating the coating at a temperature below 250 C to a pressure between 100 and 10000 bar.
U.S. Pat. No. 6,444,189 discloses a method of forming titanium dioxide particles by adding an acidic titanium salt solution to an aqueous base at a temperature of 20 C to 95 C to precipitate the particles while keeping the final pH between 2 and 4.
U.S. Pat. No. 7,224,036 discloses a method of forming a photoelectric transducer using a binder and an oxide which includes a pressure treatment at a low temperature to avoid sintering.
WO 2007/023543 discloses a method of forming a titanium oxide using a process that utilizes a titanium nitride intermediate and finished by electrolysis.
WO 2007/020485 discloses a low temperature method of forming titanium oxide photo catalysts with a dye modified surface.
U.S. Pat. No. 5,362,514 discloses a photo electric anode on which a porous metal oxide is coated and includes a porphyrin-Phthalocyanine dye.
U.S. Pat. No. 5,693,432 discloses a titanium oxide and a polymeric solid electrolyte.
U.S. Pat. No. 6,538,194 discloses a photo electrode cell including anatase titanium dioxide and a sealed electrolyte and conductive protrusions are covered by the oxide layer.
U.S. Pat. No. 6,685,909 discloses a nano crystalline titanium dioxide hetero junction materials with a shell of Molybdenum oxide.
U.S. Pat. No. 6,855,202 discloses shaped nano crystal particles including branched particles.
Patent specification WO 2004/088305 discloses the use of TiO2 photoelectrodes in determining chemical oxygen demand in water samples. For this application the TiO2 photocatalyst should possess the following general characteristics:
(i) Be readily immobilised to form the photoanode;
(ii) Readily achieve the immobilised thin-film form on a conductive substrate (photoanode) with uniformity and reproducibility;
(iii) Provide high quantum efficiency, photocatalytic activity and superior kinetic properties;
(iv) Be selective and offer highly sensitive photocatalytic oxidation towards organic compounds (over water oxidation);
(v) Provide high oxidation power, capable of rapidly mineralising a wide-spectrum of organic compounds in a non-discriminatory manner;
(vi) Having good connectivity among the crystal grain boundaries, so enabling 100% photoelectron collection efficiency;
(vii) Having stable surface properties, which eliminate the need for preconditioning before use.
(viii) Offering low photocorrosion and high mechanical adhesion to the substrate, so ensuring long-term stability.
It is an object of this invention to provide a range of fabrication methods for producing preferred photoelectrodes for various applications.